1. Field of the Invention
The present invention relates to communication systems, and, in particular, to a multiple-branch wireless receiver for multiple-antenna reception.
2. Description of the Related Art
Reliable and efficient transmission of information signals over imperfect communication channels is essential for wireless communication systems. One traditional transmission system of the prior art is illustrated in FIG. 1A in which transmitter 101 transmits, via a single antenna 103, a signal through a communication channel to a single antenna 104 of receiver 102. The signal is typically a carrier that has been modulated with data using any one of a number of different modulation techniques known in the art of communications. This type of transmission is also known as single input, single output (SISO) transmission. One characteristic of communication channels is multi-path distortion in which the transmitted signal energy is spread over several different signals, with each signal taking a slightly different period of time to reach its destination. Another characteristic of communication channels is fading and attenuation in which signal energy is lost. These communication channel characteristics contribute to signal degradation as the signal passes through the channel.
Another traditional transmission system of the prior art is shown in FIG. 1B in which transmitter 111 transmits, via a single antenna 113, a signal to receiver 112. However, receiver 112 includes 2 or more antennas 114(1) through 114(N). Each antenna detects a component of the transmitted signal, and diversity combining is used to combine the several received signals to reduce effects of signal degradation (e.g., false detection, missed detection, or bit errors). This type of transmission is also known as single input, multiple output (SIMO) transmission.
For SISO transmission, a receiver detects the transmitted signal from the communication channel and processes the received signal to extract the information. FIG. 2 shows front-end portion 200 of a SISO receiver of the prior art. The circuitry, or “receive chain,” used to translate the detected signal to baseband (BB) typically includes antenna 201 to detect the signal, filter 202 to bandpass the detected signal and filter out noise, low-noise amplifier 203 to amplify the detected signal, and mixer 204 to demodulate the signal from the carrier to BB (or near-BB). Front-end portion 200 further includes variable gain amplifier 205 to adjust the gain of the demodulated signal for filtering, filter 206 to bandpass the baseband signal, variable gain amplifier 207 to adjust the gain of the bandpass signal for sampling, and analog-to-digital converter (ADC) 208 to generate digital samples of the bandpass signal for processing by digital signal processor (DSP) 210. For SIMO transmission, a receive chain might be used similar to that shown in FIG. 2, though modified to account for delay and the combination of the several received signals. Alternatively, several different receive chains might be used, with the DSP implementing the signal combination.
For MIMO transmission, as illustrated in FIG. 1C, a transmitted sends separate signals on two or more transmit antennas, the separately transmitted signals are combined as they pass through the channel, and the receiver receives the combined signals on each of two or more receive antennas. The receiver detects and demodulates each of the transmitted signals and processes the received signals to extract the information. FIG. 3 shows MIMO receiver 300 having receive chains 301(1) through 301(N) detecting, demodulating, and sampling corresponding ones of the transmitted carrier signals for processing by DSP 302. The duplicated circuitry, or receive chains, required to translate each carrier signal to BB is termed a “multiple-branch” (or “multiple-chain”) receiver.
Referring to receive chain 301(1) of FIG. 3, bandfilter (BF) 310(1) limits the frequency spectrum of the signal from the antenna to the desired band (e.g., the Wireless LAN band, or cellular band) which might have up to several 100 MHz of bandwidth. Low-noise Amplifier (LNA) 311(1) is employed as a first amplification stage for low-power filtered signals (e.g., in the pWatt range) from antenna 302(1). LNA 311(1) might be gain-adjustable (e.g., a high- and a low-gain mode) to capture both very low as well as very large signals. Mixer 312(1) translates, in frequency, the input signal from LNA 311(1) from its RF carrier down to a much lower frequency (i.e., to an intermediate frequency (IF) or to BB). For receive chain 301(1), mixer 312(1) translates the input signal either directly to BB (“Zero IF, ZIF” or “Direct Conversion”) or to a “low IF” (LIF) frequency band not far from BB. Receive chains of other MIMO receivers of the prior art might comprise two or more mixing and IF stages. Receive chains through 301(N) operate in a manner analogous to that described above for receive chain 301(1).
First programmable gain amplifier (PGA) 313(1) pre-adjusts the BB or low-IF signal level from mixer 312(1) for channel selection filter (CSF) 314(1), since CSF 314(1) covers a certain range of signal levels. Signal-level pre-adjustment by PGA 313(1) prevents the output signal from mixer 312(1) from being so low it disappears into the noise floor or from being so high it saturates the filter circuitry. In addition, PGA 313(1) adjusts the gain of the signal that is applied to CSF 314(1) so that the signal is within the dynamic range of CSF 314(1), allowing for linear operation of CSF 314(1). While not explicitly shown in FIG. 3, signals from mixer 312(1) are typically complex valued (i.e., having in-phase and quadrature components), such that each component in the chain has two physical inputs as well as two physical outputs.
CSF 314(1) filters the signal from mixer 312(1) so as to pass the desired information channel, which, for example, in Wireless LANs operating according to the IEEE 802.11a/g OFDM standards has a bandwidth of approximately 20 MHz (e.g., typically 16-17 MHz). CSF 314(1) might have a pass-band width that is wider than that of the desired information channel, with subsequent filtering to the BB signal typically performed in the digital domain by DSP 302. Second PGA 315(1) adjusts the signal level to be within the range supported by analog-to-digital converter (ADC) 316(1). ADC 316(1) periodically time-samples the signal from PGA 315(1) into quantized levels. In Wireless LANs operating according to the IEEE 802.11a/g OFDM standards, a typical sampling period is 50 ns (i.e., corresponding to a sampling frequency of 20 Msps (Mega samples per second)). DSP 302 then processes the sample sequence from ADC 316(1), along with sample sequences from ADCs 316(2) through 316(N).
One successful approach to achieving reliable transmission is multi-carrier modulation (MCM). MCM is a modulation technique that might employ several transmit antennas at the transmitter. The principle of MCM is to divide a communication channel into a number of sub-carriers (also called tones or bins), with each sub-carrier independently modulated. Information is modulated onto a tone by varying the tone's phase, amplitude, or both. Each modulated tone is then transmitted through the communication channel, usually via a separate or corresponding antenna.
Orthogonal frequency division multiplexing (OFDM) is a form of MCM in which tone spacing is selected such that each tone is orthogonal to all other tones. OFDM wireless local area network (wireless LAN or WLAN) systems are typically designed to conform to either a contention-based wireless medium access standard such as IEEE 802.11 or a scheduled time-division duplex (TDD) wireless medium access standard such as European Telecommunications Standards Institute (ETSI) HIPERLAN/2. In a WLAN system conforming to a contention-based standard, OFDM stations compete for access to the wireless medium using “fair contention” medium-sharing mechanisms specified in the standard. In contrast, medium access in a scheduled TDD-conforming WLAN system is controlled by a single designated station, which schedules medium access for all other transceivers.
IEEE Standard 802.11 and its extensions 802.11a/b/g specify the physical layers and medium access control procedures for OFDM WLAN systems. For example, an 802.11a-compliant system operates in the 5-GHz radio-frequency band and provides data communication capabilities of 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/s. The system uses 52 tones (numbered from −26 to 26, excluding 0) that are modulated using binary or quadrature phase shift keying (BPSK/QPSK), 16-quadrature amplitude modulation (QAM), or 64-QAM. In addition, the system employs forward error correction (convolutional) coding with a coding rate of ½, ⅔, or ¾.
In a WLAN system operating according to the 802.11a/g standard, each station (i.e., a transceiver that is either an access point or a client) continuously monitors the communication channel (i.e., the wireless radio medium). Since the access protocol is “random”, the receiver of a listening station does not know when the next data packet will arrive and at what power level the packet will arrive. Therefore, the receiver detects and adjusts the signal level of an incoming packet over a relatively large signal power range (i.e., when the incoming packet has very small signal power as well as very high power). Signal power is typically measured in dBm, which is the logarithmic value 10 Log 10 (P/mW) [dBm], where P/mW is the received signal power P (measured at the antenna) normalized to 1 mW.
In WLAN systems, a typical power-level range that is covered by the receiver may be between approximately −20 dBm (=10 μW), for very strong signals from nearby transmitters and −90 dBm (=1 nW) for very weak signals from remote transmitters. This power-level range is referred to as the WLAN system's dynamic range.
In general, the dynamic range of a system, or block, is the range of signal levels at the input of the block for which, at the output of the block, the desired signal is neither hidden in the noise floor, nor saturated, as illustrated on FIG. 4. Due to thermal noise processes, every block has a certain noise level at its output. The input signal level that corresponds to the output level at the noise floor is the smallest detectable signal. The diagonal line in FIG. 4 denotes an ideal input-output signal relationship for linear operation. If the input signal level is smaller than minin, the corresponding output signal level is masked by the noise floor at the output. On the other hand, since a block has a limited maximum output level (e.g., due to the power supply voltage), at a certain point an increase of the input level does have a corresponding linear increase at the output. At this point the output signal of the block is “distorted” (the output signal level “clips” or “saturates”). The point maxin up to which the input signal does not cause the output level to clip is the upper end of the dynamic range; very often this point is characterized as the 1 dB compression point, at which the output is 1 dB less than the ideal value.
Whenever the input signal level is outside the block's dynamic range, the output signal is often no longer useful for further processing since the overall output signal is either heavily noise- or distortion-dominated. A circuit having a relatively large dynamic range generally requires complex circuitry typically occupying a relatively large (IC) chip area and exhibiting relatively large power consumption. Large dynamic range generally requires complex circuitry because more complicated and more powerful analog processing is required to maintain i) a relatively small noise level and ii) a relatively linear input-to-output relationship from low to high input signal levels. Therefore, a given implementation typically attempts to maintain the dynamic range of each block (e.g., a receive chain) at a minimum.
In a given implementation of a multiple-branch receiver, duplicating several receiver chains is expensive and requires substantial operating power. In particular, the channel selection filter, which passes the desired MCM signal from the antenna, typically occupies substantial area of the IC chip and consumes a large percentage of the operating power. Therefore, simple duplication of the channel selection filter is very undesirable in the design of a multiple-branch receiver. Unfortunately, each channel selection filter covers a large dynamic range to pass very strong signal levels from a nearby transmitter as well as very weak signals from remote transmitters, such as is typical for wireless local area networks.
Dynamic range of a block is very important for detection of a WLAN packet. Whatever signal level an arriving packet has when the receiver is in idle mode, the receive chain desirably does not mask or clip the desired signal. Masking or clipping of the output signal of a block results in failed packet detection. Packet detection is commonly accomplished by measuring certain properties of the desired signal. For example, autocorrelation properties of the packet's preamble (that is, a known “header” or training sequence transmitted at the beginning of the packet before the actual data starts) are measured with a correlator to detect the presence or absence of a packet.
A “received signal strength indication” (RSSI) module is an analog power measurement circuit that performs a square and time-average (by, e.g., low-pass filtering) of the signal input to the RSSI module. Depending on the time-constant of the low-pass filter, the output of the RSSI module is a short-term indication of the signal level sensed at the input of the RSSI module. An RSSI module is employed at various stages in an analog circuit to enable gain adjustment. If the output signal of a given circuit block (“block under test” or BuT) is monitored by an RSSI module, the RSSI module produces a relatively fine estimate of the sensed signal power. Using the estimate of sensed output signal power and the known gain of a given BuT, the input power to the BuT might be calculated, provided the BuT operates within its dynamic range. Otherwise, the RSSI level indicated by the RSSI module is a value close to the noise power level or the saturation level.
A simple RSSI measurement, such as shown in FIG. 4, uses three quantized RSSI levels. The three RSSI levels are: “Below Threshold” (BT), “Linear Range” (LIN), and “Clipped” (CL). BT indicates that the signal is very small, typically masked by the noise floor. LIN indicates the range of signals in which the BuT operates linearly (i.e., within the BuT dynamic range), and CL indicates when the signal is greater than a certain threshold (indicating, e.g., saturation of the BuT).
FIG. 5 shows format of packet 500 defined for a system operating in accordance with WLAN 802.11a/g OFDM. Packet 500 includes short preamble 501, long preamble 502, and data 503. Short preamble 501 is used by a receiver to detect a packet (i.e., to distinguish it from noise or other interference that might be present on the channel) and adjust gain settings. Long preamble 502 is used to estimate other parameters and variables such as the frequency offset, symbol and sample timing, and propagation channel characteristics (e.g., impulse response or transfer function).
Short preamble 501 is periodic. For example, the short preamble may be periodic in that the same signal repeats every 0.8 μs, with 10 repetitions. This periodicity of short preamble 501 is commonly used for a correlation operation on the signal. The correlation operation is an auto-correlation that i) delays the signal from the antenna by multiples of 0.8 μs and ii) compares (correlates) the delayed signal with the non-delayed signal. If there is a strong correlation, this suggests that there is an 802.11 a/g OFDM-compliant preamble such that the overall reception process needs to be started. In a high-data-rate MIMO system with, for example, two transmit antennas and two receive antennas, a distinguishable MIMO preamble is transmitted by each of the two antennas.
Either RSSI-level indication or correlation might be used to detect the start of a received packet, but RSSI-level indication and correlation exhibit different performance depending on the signals that are received from the antenna. For example, if there are various “rogue” signals in the channel, such as non-WLAN microwave or cordless phone signals, an RSSI-level indication might erroneously suggest the start of a received packet, leading to false detection/alarm. If, on the other hand, correlation is used, unless the signal exhibits the desired periodicity, false detection is unlikely.
Another technique of the prior art for radio signal reception from a single receiver antenna is a sub-ranging radio architecture. As described above for the MIMO system of FIG. 3, a programmable gain amplifier (e.g., amplifier 313(1)) is employed before filtering (by, e.g., filter 314(1)) to scale the signal to the allowable range (dynamic range). Under steady-state conditions (e.g., assuming the input power level is known), this scaling might be accomplished relatively accurately. However, the actual signal power and corresponding pre-amplified level of an incoming packet at the stage after initial gain adjustment (by, e.g., amplifier 311(1)) cannot be predicted. Consequently, during idle mode to detect the start of a packet preamble, the gains of the various amplifiers are set relatively high (“sensitive”) in order to ensure reception of packets with relatively weak signal levels.
However, due to the limited dynamic range of the channel selection filter, clipping might occur if the input signal is very strong. To avoid clipping, a sub-ranging radio architecture might be employed. FIG. 6 shows a prior-art sub-ranging radio receiver 600 including BF 601 to band-filter the received signal from antenna 610, LNA 602 to amplify the band-filtered signal, and mixer 603 to demodulate the amplified, band-filtered signal to BB. However, sub-ranging radio receiver 600 further includes two sub-chains 604 and 605. Sub-chain 604 includes amplifiers 607 coupled to corresponding CSF 606, and sub-chain 605 includes amplifier 609 coupled to corresponding CSF 608.
Each of sub-chains 604 and 605 is configured such that each block (amplifier and CSF) operates in a linear range for a given level of the BB signal from mixer 603. For example, one sub-chain amplifier might have relatively low gain, while the other sub-chain amplifier has relatively high gain. In the conventional receiver with one or more receive chains each covering the input signal, a relatively wide dynamic range might be required for the post-mixing channel selection filter, but when sub-ranging is employed, receive sub-chains share the overall dynamic range between each other. Consequently, each of filters 606 and 608 is relatively i) small in terms of IC chip area, ii) less complex in terms of circuit design, and iii) low-power in comparison to the post-mixing filter of, for example, 314(1) of the MIMO receiver of FIG. 3. FIG. 7 illustrates the comparison between dynamic range of a conventional architecture receiver and each sub-chain of a sub-ranging receiver.